a) Field of the Invention
The present invention relates to a high precision coordinate measuring device utilizing interference of laser light.
b) Description of the Prior Art
First, description will be made, with reference to FIG. 1, of a two-dimensional coordinate measuring device utilizing interference of laser light which is configured on the basis of the prior art. Rectangular reflecting mirrors Mx and My are fixed to a stage 1 which is displaceable in an X direction and a Y direction. These reflecting mirrors are disposed so as to be perpendicular to measuring arms of an X axis measuring laser interferometer (hereinafter referred to as an X interferometer) Ix and a Y axis measuring laser interferometer (hereinafter referred to as a Y interferometer) Iy respectively. The X interferometer Ix measures a displacement in the X direction when the stage 1 is displaced in the X direction, whereas the Y interferometer Iy measures a displacement in the Y direction when the stage 1 is displaced in the Y direction. The reference numeral 2 represents a position detector such as a position detecting microscope which detects locations of points on a FIG. 3a traced on an object to be measured 3. Optical axes of the X interferometer Ix, Y interferometer Iy and the position detector are perpendicular to one another and intersect at a point. When the interferometers and the position detector are disposed as described above, Abbe's condition is satisfied and, even if the stage 1 is slightly inclined around the X axis and/or the Y axis, measuring errors proportional to an angle of the inclination (Abbe's errors) are not produced. In this example, however, the stage 1 which is heavy and mounts the object 3 must be displaced, thereby bending a base (not shown) to which the X interferometer Ix, the Y interferometer Iy and the position detector 2 are fixed when the base does not have sufficient rigidity. As a result, distances from the interferometers to the reflecting mirrors are changed, thereby constituting a cause of measuring errors. For obtaining a high measuring accuracy on the order of 1 nanometer, it is therefore necessary to enhance rigidity of the base, which results in enlarging the measuring device and increasing a weight of the measuring device. Accordingly, a key point for enhancing the measuring accuracy lies in making weights of moving members as light as possible.
FIG. 2 is a conceptional diagram of a three-dimensional coordinate measuring device which comprises, in addition to the members of the two-dimensional coordinate measuring device shown in FIG. 1, a Z axis measuring laser interferometer Iz, or a Z interferometer Iz, and a reflecting mirror Mz disposed under the stage 1 which is configured so as to be displaceable in three X, Y and Z directions. The object 3a has a three-dimensional form. In this example also, the heavy stage 1 must be displaced and measuring errors due to the bending of the base are inevitable. A method to reduce the weights of the moving members is to dispose the interferometers Ix, Iy and Iz inside the reflecting mirrors Mx, My and Mz respectively in place of the stage 1, and integrate the interferometers with the position detector 2 so as to form a movable member. When the interferometers are disposed near the position detector, however, the interferometers strike against the object to be measured. When the optical axes of the interferometers are made higher for avoiding this striking, Abbe's condition cannot be satisfied. This inconvenience is caused also in the case of the two-dimensional coordinate measuring device.
It is said that a very high measuring accuracy on the order of 1 nanometer can be obtained with a length measuring system which utilizes interference of light. However, it is impossible to obtain the high measuring accuracy inherent in the interferometers without considering overall performance of the measuring system as a whole including the position detector. Further, the conventional coordinate measuring devices in which the heavy stages must be displaced in two directions require rigid bases which cannot be bent and inevitably have large dimensions. Furthermore, there has been unknown any means which permits measuring three-dimensional objects with high precision.